1. Field of the Invention
The present invention relates to contact lenses, more particularly to contact lenses for the correction of astigmatism. Specifically, the invention relates to a contact lens having a central optical zone which has been hardened by selectively surface treating a lens.
2. Background Technology
Astigmatism is a defect of an optical system (such as the eye) in consequence of which rays of light emanating from a point fail to meet in a focal point after passing through the optical system, resulting in a blurred and imperfect image. In the eye, this is usually the result of a mis-shaped (or toric) cornea.
Correction of astigmatism has been accomplished in the past through the use of a toric contact lens. Both rigid and soft contact lens materials have been used for making such toric lenses.
In the rigid lens category, a toric back surface corneal contact lens was originally (and principally) used to accomodate a toric cornea; not to correct astigmatic refractive error. It has been found possible to correct astigmatism by orientation of the meridional properties of a lens to properly orient its cylinder. For example, a bitoric corneal contact lens employs an additional toric surface on the front of the lens, thereby assuring proper fit by the base curve's toricity and correcting the astigmatism through the optical properties of the front curve's toricity.
Soft or hydrogel lenses have also been designed to correct astigmatism. Due to the flexibility of the lens material, these lenses tend to conform (although not perfectly) to the shape of the cornea on which they are worn. Introduction of a toric curve on the front or the base surface of a lens (but not on both surfaces) has been successful, since the entire lens assumes the cornea's shape, leaving the single surface's toricity to coact with such assumed shape, providing the functional equivalent of a rigid bitoric lens.
Both in the rigid and soft toric lenses of the past, rotational stability was required in order to maintain the orientation of the lens' cylinder vis a vis the cornea. Thus, a variety of lens orientation techniques have been developed. These include prism ballast (a thickened base portion), single and double truncations (removal of a segment at the apex or ballast of the lens), and slab-off (selective reduction of edge thickness).
While the above-described lenses have enjoyed a certain amount of success, they have also suffered from a number of deficiencies. Providing two powers on the optical curve and coordinating those powers with the rotational stabilizing means has made such lenses very difficult and expensive to manufacture, often resulting in very low yields. Another problem with prior toric corneal lenses is discomfort. This is usually due to the thickness of the lens (the lenses have to be made thicker to accomodate the mis-shaped cornea and to incorporate prism for rotational stability).
Another technique for treating astigmatism with contact lenses is the combination "hard/soft" concept. This attempts to combine the comfort benefits of a soft lens with the shape retaining characteristics of a rigid lens. Thus, the optical zone of the lens is formed of a hard material approximating the shape of a "perfect" cornea on the base curve. The tear layer entrapped between the lens and the cornea serves as a refracting medium having the shape defined by the base curve, thereby correcting the astigmatic error of the mis-shaped cornea below. At the same time, the softer, thinner periphery of the lens conforms to the cornea and supports the optical zone in position, resulting in greater comfort for the wearer. The front curve of the lens can also be selected to correct for other refractive errors. A principal advantage of these lenses is that they do not require the rotational stabilizing means described above. Improved user comfort is another significant advantage.
Some multi-stage processes for preparing "hard/soft" corneal contact lenses are described in U.S. Pat. Nos. 3,489,491 (Creighton); 3,619,044 (Kamath); 3,944,347 (Barkdoll et al.); and 4,093,361 (Erickson et al.).
The "hard/soft" lens, like the toric lenses described above, suffers from certain disadvantages. These disadvantages arise primarily in the manufacture of the lens. It is very difficult to fuse the two materials together to give a lens without unacceptable boundaries. Even when the fusion process is successful, these lenses are still difficult and expensive to manufacture. Also, the known techniques for making "hard/soft" lenses are not adaptable to the manufacture of a molded lens. In addition to these manufacturing disadvantages, the center thickness of such lenses has been noted as a cause of discomfort, and therefore, another disadvantage.
For a more detailed description of astigmatism and the corneal contact lenses which have been used for its correction, see Contact Lens Practice, Third Edition, by Robert B. Mandell, O.D., Ph.D. (Published by Charles C. Thomas, 1981).
3. Aspects of the Invention
In one aspect, the present invention provides a corneal contact lens for the correction of astigmatism, such lens being thinner than prior astigmatism-correcting lenses for increased comfort and oxygen transmission in the eye of its wearer.
In another aspect, the invention provides a comfortable corneal contact lens for the correction of astigmatism, which lens is easy and inexpensive to manufacture.
Still another aspect of the invention provides a method of treating a soft corneal contact lens replica to give additional rigidity characteristics to a central portion thereof, so that it can be used to correct astigmatism.
Yet another aspect of the invention provides a non-fused contact lens without any rotational stabilizing means, for the correction of astigmatism.